1. Field of the Invention
This invention generally relates to magnetic tape cassettes in which a tape-shaped record medium is accommodated in a wound form and, more particularly, is directed to a magnetic tape cassette in which a recorded signal can be prevented from being inadvertently or unintentionally erased by the use of a protective plug slidable in the thickness direction of a cassette casing.
2. Description of the Prior Art
In the prior art, the tape cassette is provided with a mis-erase preventing means so that a signal recorded on a magnetic tape accommodated therein can be prevented from being erased inadvertently.
As the erasure preventing means, such type is proposed, in which a protruding preventing tab is integrally formed with the tape cassette and this protruded tab is cut away to avoid the erasure. In this type of mis-erase preventing means, the protruded tab, once cut away, cannot be reattached to the tape cassette. Further, if the cutaway portion is not closed by another member and is left opened, a new signal cannot be recorded on the magnetic tape. To solve the aforenoted problem, a switching type of erasure preventing means is proposed, in which a cassette casing is provided with a sliding member slidable in the thickness direction of the cassette casing and this sliding member is selectively slid between an erase possible position and an erasure preventing position to thereby prevent the recorded signal from being erased inadvertently.
Japanese Patent Laid-Open Gazette No. 1-102788 describes the above-mentioned sliding member switching type of erasure preventing means. In this sliding member switching type of erasure preventing means, first and second engaging convex portions are formed on the sliding member at its position displaced from the sliding direction of the sliding member in an opposing relation to each other with respect to the sliding member. Also, opposing first and second engaged concave portions are formed in insertion apertures of the cassette casing at the positions coincident with the sliding direction of the sliding member. Thus, when the first engaging convex portion is engaged with the first engaged concave portion, the sliding member is brought in to the first position. Further, when the second engaging convex portion is engaged with the second engaged concave portion, the sliding member is brought in to the second position, whereby the tape cassette is set in the erase possible state and in the erasure preventing state.
According to the thus constructed conventional erasure preventing means of the sliding direction of the sliding member, the tape cassette can be repeatedly set in the erase possible condition and in the erasure preventing condition by switching the sliding of the sliding member, and by engaging the engaging convex portions with the engaged concave portions at the respective switching positions, the sliding member can be engaged at the two switching positions, ensuring that the two conditions can be detected. However, this sliding member switching slide operation encounters with the following problem. That is, under the condition that the first and second convex portions of the sliding member are disengaged from the first and second engaged concave portions of the insertion apertures or under the condition that the two engaging convex portions are disengaged from the two engaged convex portions and brought in contact with the inner wall surface of the insertion apertures, the two engaging convex portions are considerably urged against the inner wall surface to thereby hinder the sliding member from being slid smoothly. Accordingly, the sliding member must be pushed by a very large pushing force. There is then the substantial risk that the sliding member and the cassette casing may be damaged.